The present invention is related to optical devices. More particularly, it is concerned with overcoated grating couplers for use in coupling free electromagnetic radiation to surface electromagnetic waves generated in the grating.
It is well known that in the visible region of the electromagnetic spectrum it is possible to couple essentially all of the bulk electromagnetic wave incident to a metal surface into a surface electromagnetic wave (SEW) or surface plasmon wave within the metal. (For a detailed discussion of surface plasmons see, for example, E. Burstein et al., J. Vac. Sci. Technol., Vol. 11, No. 6, pp. 1004-1019, 1974.)
However, as the wavelength of the incident radiation increases into the infrared region of the spectrum and beyond, efficient coupling between the incident radiation and the surface plasmon becomes more difficult. This is due to increasing delocalization of the electromagnetic field away from the interface and the corresponding decrease in the proportion of the field within the metal as wavelength increases.
Z. Schlesinger et al. have discussed in Solid State Commun. Vol. 38, pp. 1035-1039, 1981, the phenomenon of coupling between far infrared radiation and surface plasmons in germanium coated gold or lead surfaces. In a doctoral dissertation submitted to the University of Missouri at Rolla in 1975 and entitled, "Excitation of Surface Electromagnetic Waves at Microwave Frequencies Using Optical Techniques," M. Davarpanah has described the coupling of microwave radiation into a coated metal surface in devices which employ metal rods placed over the coating.